A novel iron-sulfur flavoprotein was first discovered in the methane-producing archaeon Methanosarcina thermophila. Since this discovery, evidence has accumulated strongly indicating that this electron carrier is widely distributed in metabolically diverse procaryotes that span both the Bacteria and Archaea domains. I plan to biochemically characterize this novel flavoprotein to learn how the protein environment controls the redox potential of the FMN. I will use site-directed replacement of residues and characterize the variants using both biochemical and biophysical methods to determine how the selected residues influence properties of the FMN. I also plan to further examine the physiological role of Isf by determining the natural electron acceptor. The results are expected to lead to a better understanding of the role for this electron carrier in all procaryotes and the biochemistry of flavoproteins in general.